It has long been known that cyclohexane dicarboxylic acids can be prepared by the hydrogenation of phthalic acids over a suitable catalyst. For example, Freifelder, et al., J. Org. Chem., 31, 3438 (1966) teaches the low pressure hydrogenation of benzene polycarboxylic acids using a rhodium catalyst supported on carbon or on alumina. Good yields were obtained of the corresponding cyclohexane dicarboxylic acids by hydrogenation in aqueous solution of phthalic, isophthalic and terephthalic acids at reaction temperatures of 60.degree.-70.degree. C. and pressures less than 3 atmospheres. At this temperature range, sufficient starting acid dissolved to allow uptake of hydrogen to proceed at a good rate. For example, pyromellitic acid was hydrogenated to cyclohexane-1,2,4,5-tetracarboxylic acid with 75% yield with uptake of hydrogen complete in 1.5 hours. Yields from respective acids were: cyclohexane-1,2-dicarboxylic acid 93%, cyclohexane-1,3-dicarboxylic acid 96% and cyclohexane-1,4-dicarboxylic acid over 90%. However, the process of Freifelder required extremely high concentrations of 5% rhodium-on-carbon catalysts. Requirements were 5 grams of catalyst to 12.7 grams of pyromellitic acid, and 4.0 grams of 5% rhodium on alumina per 8.3 g (0.05 mole) of the other dibasic acids although in some instances 2.0 grams of 2.5% rhodium on alumina were used as catalyst. Freifelder noted that the success of the hydrogenation reaction could be more dependent on the water solubility of the reduction product than that of the starting acid. Freifelder was accordingly suggesting that choice of solvent could be determining the reaction rate.
Other previous teachings relate to hydrogenation of cyclic compounds. U.S. Pat. No. 2,675,390 teaches the hydrogenation of cyclic compounds at room temperature and atmospheric pressure using catalysts comprising rhodium supported upon a suitable carrier which can be carbon or alumina. Examples of cyclic compounds are benzene, toluene, benzoic acid, phthalic acid, furane, fuoric acid, phenol pyrrole, and hydroquinone in a suitable solvent, water or acetic acid. Yields of practically 100% were obtained by hydrogenating pyrroles. High concentrations of catalyst to reactant material were required. For example, as high as 1 gram of catalyst was required per 0.5 cc of diethyl phthalate in acetic acid. U.S. Pat. No. 2,828,335 teaches hydrogenation of phthalic acid salts in water in the presence of a catalyst containing ruthenium in an amount of from 0.01 to 5 weight percent at a temperature within the range of from atmospheric to 250.degree. C. Yields from sodium isophthalate were 90 to 92 mole percent as the hexahydroisophthalic acid using low concentrations of ruthenium catalyst. Example 7 teaches a slurry of 156 grams of sodium isophthalate in 210 grams of water in the presence of 1.01 grams of ruthenium oxide. A temperature of 110.degree. C. was held for 4.7 hours to complete the hydrogenation. U.S. Pat. No. 2,888,484 teaches use of an inert liquid medium in which a phthalic acid, e.g., terephthalic acid, is at least partially soluble under reaction conditions of 150.degree. C. to 300.degree. C. and pressure greater than 1000 psig. Hexahydroterephthalic acid was obtained in yields of 93% at a temperature of 180.degree. C. and reaction pressure of 5000 psi wherein the terephthalic acid:catalyst weight ratio was 10:1. The catalyst is palladium or ruthenium on carbon or silica gel. A 5% rhodium-on-carbon catalyst at 300.degree. C. and 5000 psig for 41/2 hours gave a conversion of 100% but a yield of only 38%. At less drastic conditions, the yield was lower. U.S. Pat. No. 3,444,237 teaches hydrogenation of an alkali salt of trimellitic anhydride to yield mixed isomers of cyclohexane-1,2,4-tricarboxylic acid. Catalyst is ruthenium on activated carbon at a rate of 2 to 25 grams of catalyst per mole of trimellitic anhydride. Examples I and II teach use of 10 grams of catalyst per 200 grams (1.04 mole) of trimellitic anhydride.
Accordingly, previous investigators have determined that phthalic acids can be catalytically hydrogenated in the presence of a solvent wherein catalyst:reactant weight ratio can be in the range from 1:2 to 1:10 or higher, depending upon whether rhodium, ruthenium or palladium is used as the catalyst. Rhodium, palladium and ruthenium on activated carbon or silica have been the catalysts of choice, depending on the phthalic acid to be hydrogenated. The solution has been of water or acetic acid. Reaction rates have been increased by increasing catalyst concentration relative to reactant, although Friefelder suggested that choice of solvent could be a factor in success of the reaction.
Increasing reaction rates by increasing the amount of catalyst to reactant in a commercial process using a rhodium, palladium or ruthenium catalyst increases the economic cost of the process and can result in the process being of little economic value. Consequently, an improved process is very much desired for hydrogenating phthalic acids to cyclohexane dicarboxylic acids wherein rhodium catalyst concentration relative to parts of phthalic acid is less than previously taught, reaction rates are increased, and conversion and selectivity are both nearly 100% to obtain yields of approximately 100%.
My invention is an improvement in the preparation of cyclohexane dicarboxylic acids wherein reaction rates are increased by isolating a portion of the product stream and returning the said portion to the reactor. I have discovered that the increased concentration of product in the reactor increases significantly the reaction rate both in terms of rate of phthalic acid conversion and cyclohexane dicarboxylic acid production. It has also been found that in continuous operation, under conditions wherein the used catalyst is recycled, the used catalyst retains its catalytic activity for long periods of use.
It is accordingly an object of this invention to provide a process for the hydrogenation of phthalic acids wherein an increased rate of reaction is obtained without use of added quantities of catalyst over rates obtained by previously-taught processes using rhodium-on-carbon catalysts. It is also an object of this invention to provide a process for hydrogenation of phthalic acids wherein the catalyst is recycled for long periods of time without loss of activity. It is also an object of this invention to provide a process for hydrogenation of phthalic acids wherein a reduced weight ratio of rhodium-on-carbon catalyst to phthalic acid is used versus weight ratios of catalyst to phthalic acid in previously taught processes. Other objects will appear in further reading.